This invention relates to a method and an apparatus for manufacturing cathode-ray tubes, and more ticularly to a method and an apparatus for manufacturing cathode-ray tubes, in which a front film such as silicate film formed on a front glass panel of a cathode-ray tube by a spraying or coating operation is baked so as to be fixed to the front glass panel.
In order to bake a silicate film in this manner, it is generally necessary that the cathoderay tube be heated to not lower than 100.degree. C. In such a case, a furnace (generally called "a tunnel furnace") shown in FIG. 1 is employed in a conventional method of this kind. A furnace body 1 consists of an outer furnace frame 2 and an inner furnace frame 3. In a hot air circulating passage 4 between these outer and inner furnace frames 2, 3, a fan 5 and a heater 6 are provided in the upper and side portions, respectively, thereof. The cathode-ray tubes 7 are retained by holders (not shown) and transferred in the inner furnace frame 3 in the furnace body 1.
An apparatus of this kind is also disclosed in, for example, U.S.P. No. 4,350,514 which is corresponded to Japanese Patent Laid-Open No. 162451/1981.
The apparatus disclosed in this publication has a construction as shown in FIG. 2. This drawing is a sectional view of a heating furnace in the apparatus. In this furnace, a hot heating air (which will hereinafter be called "furnace air") is sucked by a fan 5 and sent into an inner shell 3 from the ceiling thereof and the lower side of a mesh belt 26 through a hot air circulating duct 4. This arrangement is used generally in practice so as to set a uniform temperature distribution in the furnace, and the fan 5 is called a side fan. This fan is provided on the ceiling in some cases (refer to FIG. 1). Reference numeral 28 denotes an external air supply pipe, which is disposed so that hot air can be sent into the duct 4 through the interior of an outer shell 2. The external hot air enters the furnace from the external air supply pipe 28, and is mixed with the furnace air, the resultant mixed air being blown in a heated state into the inner shell 3 through air vents 30. Reference numeral 7 denotes cathode-ray tubes, 22 a suction duct, 23 discharge ports for returning the furnace air to the interior of the duct 4, 24 a motor for driving the fan 5, and 21 a flow meter for measuring the flow rate of the external air.
In a tunnel furnace used in a convection heating method according to the above conventional techniques, it is necessary that a cathode-ray tube as a whole be inserted in the tunnel furnace and heated so as to circulate the hot air in the furnace. If these requirements are met, not only a front glass panel but also other parts of the cathode-ray tube, such the funnel glass is heated.
In order to secure the strength of a silicate film formed on the front glass panel, the film has to be baked at a temperature of not lower than 130.degree. C. However, a cathode-ray tube has two portions which are not heat resistant. One of these two portions is a base of a plastic material, and the other a gas (such as H.sub.2 O, CO, CO.sub.2, etc.) deposited on the inner surface of the cathode-ray tube.
The base is made of plastic materials. If the base is left in the atmosphere of 130.degree. C. for a long period of time (for example, thirty minutes), it is generally melted. When the gas deposited on the inner surface of the cathode-ray tube is heated, it leaves the inner surface, to cause the pressure in the cathode-ray tube to be momentarily higher. The resultant gas is deposited on the surface of the cathode and contaminates it, so that the electron emission capacity of the cathode lowers.
As mentioned above, in the so-called tunnel furnace used in a conventional convection heating method, the temperature of the front glass panel cannot be increased to a required level during a baking operation, whereby the strength of the silicate film cannot be secured.